The present invention relates generally to the control of electric motors and, more particularly, to a scheme for controlling an alternating current (AC) induction motor using a load commutated inverter circuit (LCI) for supplying electrical power to the motor.
It is common, in the discipline of motor drives, to employ power converter systems to furnish electrical power from a source to the motor. These power converters are of various types but, in the higher voltage and power ranges, are often comprised of thyristors in a bridge arrangement. The thyristors of the bridge are selectively gated in what is commonly known as phase control to vary or control the electrical power supplied to the motor. In adjustable speed AC motor drives, it is common to use two such converters, the first of which serves to convert AC power from a source (e.g., power lines) to direct current (DC) power. The second converter which is connected to the first by way of a DC link circuit serves to convert the DC power supplied thereto into variable frequency AC power which is supplied to the motor. The first (source side) converter is controlled to vary the amount of current which is furnished to the motor while the second (load side) converter--commonly called an inverter--is used to vary the frequency of the power that is supplied to the motor.
The source side converter for a typical three phase system is a six-thyristor bridge which is phase controlled to vary the output current or voltage. The load side converter, or inverter, is usually one of two general types, a load commutated inverter or a forced commutated inverter. By commutated, it is meant rendering the thyristors of the bridge non-conductive.
As is generally known in the art, to render a thyristor non-conductive, the current within the thyristor must be reduced to essentially zero value as by placing a reverse voltage across the thyristor. In a source side converter, this is normally not a problem since the converter acts basically as a rectifier and the gating on of one thyristor will effect a commutation of the then conducting thyristor since the AC source (e.g., power lines) to which the converter is connected will inherently develop the reverse voltage. In the load side converter which is supplied with DC power, however, it is commonly known that some form of reactive volt-amperes (VARs) must be present in order to effect commutation. In the load commutated inverter, these VARs are derived from the load. In the force commutated inverter there is some means such as a capacitor which is appropriately charged and at the proper time the charge on that capacitor is used to commutate the inverter.
In the discipline of variable speed AC motor drives, at lower voltages (e.g., below 1000 volts) the preferred drive has used a cage type induction motor which is supplied by a forced commutated inverter such as a square wave inverter, a pulse width modulated inverter or a current source inverter. The induction motor is preferred because of its simplicity and its more rugged nature. These advantages of the induction motor have overcome the disadvantages of the necessary commutation circuitry of a forced commutated inverter. This technology, however, has been generally limited to inverters in which a single main or power thyristor switching device is provided in each inverter leg because of the technical difficulties associated with parallel or series connected thyristors required for higher powered ratings. For example, it is extremely difficult to get precise simultaneous forced commutation of multiple thyristors included in series in one leg of a bridge where the voltage requirements exceed those of a single thyristor. At higher voltages (e.g. above 1000 volts), therefore, it has been the practice to use a load commutated inverter to supply a synchronous motor.
A load commutated inverter system having both source side and load side converters of the type mentioned above is normally regarded as suited to pass real current from one AC source to another (or a load) while drawing a lagging reactive current from both its source and load sides. Thus, as a motor drive, it is usually used to drive a synchronous motor and not an induction motor. This is because a synchronous motor, which has its rotor excited by a DC current, is able to serve as a source of lagging reactive current to the load side of the LCI and thus effect commutation. The LCI is not usually regarded as a suitable source for driving an induction motor because the motor, like the LCI, requires a source of lagging reactive current at its terminals. Thus, while the load commutated inverter or LCI is less expensive and capable of handling higher voltages, it has not been deemed suitable for use with the simpler and less expensive induction motor.